A hybrid vehicle is driven by a drive source that is a combination of an engine and a motor capable of electricity generation (motor generator), and such a vehicle has been developed for improving fuel efficiency and for reducing exhaust gas in environmental protection. Typically, a hybrid vehicle is controlled for stopping the engine when the vehicle stops in so-called idling-elimination. In this case, when the engine stops, the mechanically driven oil pump, which has been in operation for supplying hydraulic oil, for example, to the ratio-change mechanism, also stops. Therefore, for complementarity, an electrically driven oil pump or electrical oil pump is provided to supply hydraulic oil only while the engine is not in operation under the idling-elimination control, the electrical oil pump being driven by an electrical motor, which receives electric power from the battery (refer to, for example, Japanese Laid-Open Patent Publication No. 2003-307271).
In this case, the required hydraulic pressure applied with oil by the electrical oil pump is achieved by controlling the torque of the electrical motor, which drives the pump. However, the hydraulic pressure applied by the electrical oil pump is dependent on the viscosity of the oil. If the viscosity changes as the temperature of the oil changes, then the hydraulic pressure cannot be achieved appropriately by the electrical oil pump. For this reason, there is known a hydraulic pressurizer system that controls to cancel or prohibit stop-engine-idling control if the temperature of the oil is not within a predetermined range and supplies hydraulic oil only from the mechanical oil pump, which is being driven continuously by the engine without any interruption from the idling-stop control (refer to, for example, Japanese Laid-Open Patent Publication No. 2000-104587).
However, if the idling-stop control is always cancelled or prohibited while the temperature of the hydraulic oil is out of the range, then the effectiveness of idling-elimination is limited by the performance of the electrical oil pump or by the temperature range for the operation of the electrical oil pump in a case where this range is narrower than the temperature range for the operation of the automatic ratio-change mechanism. As manufactured products, electrical oil pumps and ratio-change mechanisms often have different or individual performances, respectively, because of their uneven product qualities or of their individual aging. On this background, if the idling-stop control is to be cancelled or prohibited depending on the oil temperature in all cases, then the systems must be designed to meet the lowest performance possible in the operation (or the narrowest temperature range for the operation) of electrical oil pumps and ratio-change mechanisms. This leads to a problem of inefficient performance by the hydraulic pressurizer system in idling-elimination.